Date: Fri, 21 Jul 2017 18:42:35 +0000 (UTC) From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r321349 - head/sys/ufs/ffs Message-ID: <201707211842.v6LIgZEZ066837@repo.freebsd.org>
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Author: kib Date: Fri Jul 21 18:42:35 2017 New Revision: 321349 URL: https://svnweb.freebsd.org/changeset/base/321349 Log: Improve publication of the newly allocated snapdata. For freshly allocated snapdata, Lock sn_lock in advance, so si_snapdata readers see the locked snapdata and not race. For existing snapdata, if the thread was put to sleep waiting for sn_lock, re-read si_snapdata. This either closes the race or makes the reliance on LK_DRAIN less important. Reported and tested by: pho Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Modified: head/sys/ufs/ffs/ffs_snapshot.c Modified: head/sys/ufs/ffs/ffs_snapshot.c ============================================================================== --- head/sys/ufs/ffs/ffs_snapshot.c Fri Jul 21 18:36:17 2017 (r321348) +++ head/sys/ufs/ffs/ffs_snapshot.c Fri Jul 21 18:42:35 2017 (r321349) @@ -2638,8 +2638,8 @@ try_free_snapdata(struct vnode *devvp) static struct snapdata * ffs_snapdata_acquire(struct vnode *devvp) { - struct snapdata *nsn; - struct snapdata *sn; + struct snapdata *nsn, *sn; + int error; /* * Allocate a free snapdata. This is done before acquiring the @@ -2647,23 +2647,37 @@ ffs_snapdata_acquire(struct vnode *devvp) * held. */ nsn = ffs_snapdata_alloc(); - /* - * If there snapshots already exist on this filesystem grab a - * reference to the shared lock. Otherwise this is the first - * snapshot on this filesystem and we need to use our - * pre-allocated snapdata. - */ - VI_LOCK(devvp); - if (devvp->v_rdev->si_snapdata == NULL) { - devvp->v_rdev->si_snapdata = nsn; - nsn = NULL; + + for (;;) { + VI_LOCK(devvp); + sn = devvp->v_rdev->si_snapdata; + if (sn == NULL) { + /* + * This is the first snapshot on this + * filesystem and we use our pre-allocated + * snapdata. Publish sn with the sn_lock + * owned by us, to avoid the race. + */ + error = lockmgr(&nsn->sn_lock, LK_EXCLUSIVE | + LK_NOWAIT, NULL); + if (error != 0) + panic("leaked sn, lockmgr error %d", error); + sn = devvp->v_rdev->si_snapdata = nsn; + VI_UNLOCK(devvp); + nsn = NULL; + break; + } + + /* + * There is a snapshots which already exists on this + * filesystem, grab a reference to the common lock. + */ + error = lockmgr(&sn->sn_lock, LK_INTERLOCK | + LK_EXCLUSIVE | LK_SLEEPFAIL, VI_MTX(devvp)); + if (error == 0) + break; } - sn = devvp->v_rdev->si_snapdata; - /* - * Acquire the snapshot lock. - */ - lockmgr(&sn->sn_lock, - LK_INTERLOCK | LK_EXCLUSIVE | LK_RETRY, VI_MTX(devvp)); + /* * Free any unused snapdata. */
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